In many scientific applications, there is an urgent need to cool a large plurality of sensors, or individual sensors to very low temperatures, and to maintain the sensors stable at the desired temperature over a long time period. For example, in one scientific development it was desired to include a total of 8,192 visible light photon counter detectors in a single package. It was necessary that the detectors be maintained for periods up to six months at 6.5.+-.0.2 K. (degrees Kelvin). To achieve this objective, it was planned to contain the detectors in a single cryostat using cold helium as a temperature controlling medium. Desirably, each detector would be maintained at the same temperature.
In prior art cryostats, liquid helium is delivered to a region near the object or objects, which are being cooled. The liquid helium is then heated to the desired temperature as determined by a temperature sensor near the cooled object. The helium directly by contact or indirectly cools the object. Such a construction requires a reasonably steady flow of helium and a good automatic temperature control system. When multiple objects, e.g., detectors, are to be cooled, it is generally necessary in the prior art to have multiple temperature sensors and either a heat controller or helium flow controller, or both, at each detector to maintain steady temperatures.
What is needed is a cryostat construction that maintains multiple objects to be cooled at the same stable temperature using only a single refrigerant flow controller and a single heater and associated controller.
FIG. 1 schematically illustrates a liquid helium cooled cryostat, which is typical of a number of presently available cryostats, e.g., HELI-TRAN Model LT-3-110, manufactured by APD Cryogenics Inc. of Allentown, Pa., USA. In this prior art construction, liquid helium is delivered through a vacuum insulated transfer line 12 from a pressurized (3 PSIG) storage dewar (not shown) to a cold stage 14 within a vacuum insulated housing 18 to which a sample 16, e.g., a visible light photon counter detector, is attached. The liquid helium is delivered at a temperature of 4.2 K., its normal boiling temperature at atmospheric pressure, and the liquid is heated by a heater 20 that is positioned between the discharge from the transfer line 12 and the sample 16. Because the liquid helium is delivered from the transfer tube at 4.2 K., it is possible by adjusting the heater 20 to maintain the sample 16 at a desired higher cold stage temperature of 6.5 K. The sample 16 is maintained at the desired temperature, as measured by a temperature sensor 22, which is mounted close to the sample 16 and is used in controlling both the helium flow rate and the energy input to the heater 20.
After cooling the sample 16, it is conventional to have the cold helium flow through a second heat exchanger, that is, a thermal intercept 24, in order to cool a radiation shield 26 and to intercept heat that flows to the cold stage 14 through electrical leads and structural supports.
FIG. 2 illustrates a variation of this cooling and control method wherein similar elements have been given the same reference numerals as in FIG. 1. Helium gas leaving the transfer tube 12 at the cold stage 14 is heated by the cold stage heater 20 and flows over the sample 16 to directly cool the sample by convection and conduction. Typically, the cold helium then reverses direction and flows through a thermal intercept heat exchanger 24 to cool a radiation shield 26 in the housing 18 and to intercept heat entering the system from other sources. Such a construction is incorporated, e.g., in HELI-TRAN Model LT-3-110, mentioned above.
In another construction (FIG. 3), liquid helium is used to maintain a copper block 28 at 4.4 K. by conduction from a mass of stored liquid helium 30. As heat is transferred into the mass 30, there is a boil off of helium gas through a vent line 32 so as to intercept heat leakage toward the cold region. A sample 34, e.g., a cassette holding multiple visible light photon counter detectors, is mounted in a well 36 in the copper block 28, but the sample 34 is not in contact with the copper block 28. An electric heater and an associated temperature sensor are located adjacent to the inserted end 38 of the sample 34. Helium gas, boiled from the liquid mass 30, fills the space between the copper block and the sample 34 to transfer heat generated in the sample 34 to the copper block 28.
The sample 34 is maintained at a desired temperature that is higher than the temperature of the liquid helium. There is some heat leakage through the support structure and electrical leads toward the copper block 28. The heater at the end 38 of the sample 34 provides the balance of energy that is needed to maintain the sample at an elevated temperature, e.g., 6.5 K. The position of the sample 34 in the well 36 is adjusted to minimize the heat that must be added by means of the heater. Helium gas in the vent line 32 flows away from the sample 34 toward a discharge port (not shown) as is indicated by the arrow 39. In this flow path, heat that enters through the cryostat walls is absorbed and shielded from the sample 34 by a cooling heat exchanger 37 such that maintenance of the desired sample temperature is not significantly affected by the external ambient of the cryostat.
The construction of FIG. 3 has the disadvantages of requiring a heater, temperature sensor and temperature controller for each of the samples 34 that may be mounted in the common copper block 28. In some prior art constructions, it is also necessary to have multiple controls of helium flow rate for different portions of a multiple detector device. Also, in the prior art a significant amount of heat flows into and out of the copper block 28, which serves as a heat sink for different sources, so that temperature gradients in the copper block 28 are significant. Therefore, the temperature differential between the hottest and the coldest sample 34 in the same block 28 may be beyond limits which are tolerable in devices using a plurality of samples, e.g., detectors. Thus, it is extremely important in a construction for multiple samples that the temperature of the heat sink be held at a very uniform and constant temperature.